A conventional vehicle is provided with a transmission system between the internal combustion engine and driving wheels. A conventional transmission system varies the driving force and the speed of wheels in accordance with widely variable driving conditions of the vehicle itself to allow the engine to fully exert its own performance. In conjunction with the transmission system, there are a variety of continuously variable transmission systems like those which are disclosed in official publications of Japanese Patent Laid-Open Nos. 57-186656 (1982), 59-43249 (1984), 59-77159 (1984), and 61-233256 (1986) for example. Any of these conventional continuously variable transmission systems transmits the driving force by either expanding or contracting the radius of rotation of a belt by either expanding or contracting the width of channel formed between a stationary pulley member secured to a revolving shaft and a movable pulley member mounted axially movably on the revolving shaft so that the movable pulley member can come into contact with and leave the stationary pulley member for varying the belt drive ratio.
Conventional continuously variable transmission systems leave the following problem. In drive mode, a conventional continuously variable transmission system executes control of the objective line pressure by determining the objective line pressure in anticipation of a certain safety margin value when an oil-pressure-driven clutch does not slip itself, while the actual line pressure is controlled to follow the objective line pressure by applying closed-loop control means. A convention line pressure control system sets the objective line pressure to allow the oil-pressure-driven clutch to slip itself before the pulley belt slips, and as a result, the belt can be prevented from incurring wear.
Nevertheless, when operating the line-pressure control system mentioned above, even though the line pressure is controlled to follow the objective clutch pressure, due to variable clutch characteristics or differences between different clutch units, clutch slipping may still take place. In addition, a certain difference can be generated between the objective line pressure and the actual one due to the presence of a variety of uncertain factors, including an unsatisfactory initial characteristic of the pressure sensor detecting the clutch pressure in contrast with the designed value, degradation of such characteristic after years of service, or faulty symptoms caused by external stress, thus eventually generating unwanted slipping of the clutch itself. To solve those problems, conventional line-pressure control systems introduce a substantial safety margin to the objective line pressure.
On the other hand, conventional line-pressure control systems need to use closed-loop control so that the actual line pressure can match the objective line pressure which is provided at a high level. This in turn results in an economic and practical disadvantage.
A primary object of the invention is to overcome those problems mentioned above by providing a novel method and apparatus for controlling line pressure for use with a continuously variable transmission system related to the invention. The invention provides the objective line pressure with a substantially low safety margin without unnecessarily causing line pressure to rise at the initial setting by preventing the entire system from becoming complicated and expensive, in which the control operation is executed so that the line pressure can rise by a specific amount in the presence of the predetermined conditions when the system detects occurrence of slipping of clutch during drive mode, thus securely implementing an ideal and optimum clutch engagement.
To achieve the above object, the invention provides a novel method and apparatus for controlling line pressure of a continuously variable transmission system such transmission system being controlled by either expanding or contracting the width of the channel between a stationary pulley member and a movable pulley member which is contactably and removably set with respect to said stationary pulley member, in order to either expand or contract the radius of the rotation of a belt wound on said pulley members for eventually varying the belt ratio, in which the line-pressure control system causes the line pressure to rise by a specific amount in the presence of predetermined conditions when it detects slipping of the clutch while the drive mode is underway, so that an optimum clutch engagement can securely be implemented.
Advantages of the invention include the following. According to the invention, when the control system detects slipping of the clutch while the drive mode is underway, it causes the line pressure to rise by a specific amount to effectively prevent the clutch from slipping. The invention effectively prevents the clutch from slipping and provides the objective line pressure with a substantially low safety margin, thus securely preventing the line pressure from unnecessarily rising at the initial setting. This in turn minimizes the cost of the entire system.